Creating login & logout button

I am just starting with Silverstripe and I am having a (very simple?) problem. What I want to do is simply create a login / logout button somewhere on the site, the default menubar would be a good place. This should be very easy I guess, but I don't know how to do it...

On the back-end I could simply create a new menu-item which refers to /Security/login, but then it will still say "Login" after logging it, I would like it to disappear or change to logout. What is the best and simplest way to realize this?

I have searched the SilverStripe documentation, but I couldn't find it. Can somebody please explain how this works or provide me a link to a page where it is described? Thanks in advance! :-D